1. Field of the Invention
The present invention relates to a sintered silicon nitride-based body which can be used advantageously for cutting tools, wear-resistant tools, and parts for mechanical structures required to have a high strength and a high toughness at a high temperature, and to a process for producing the same.
2. Description of the Prior Art
Sintered silicon nitride-based bodies are expected as materials for cutting tools, wear-resistant tools, parts for mechanical structures and wear-resistant parts required to have a heat resistance and a high strength.
Silicon nitride as such is poor in sinterability and a sintering aid component such as Y.sub.2 O.sub.3 or Al.sub.2 O.sub.3 is thus added thereto conventionally. Mass transfer due to the formation of a liquid phase of the aid component results in a densified sintered silicon nitride body. However, since a major part of the liquid-phase forming component other than Si.sub.3 N.sub.4 is converted into a glass phase and forms a grain boundary phase with the typical liquid phase sintering alone, the glass phase is softened to reduce the strength significantly under a high temperature of 1200.degree. C. or higher.
For example, according to Japanese Patent Publication No. 56-28865, an Si.sub.3 N.sub.4.Y.sub.2 O.sub.3 crystal phase is precipitated in the grain boundary during sintering. A bending strength at 1300.degree. C. is as small as about 75-93 kg/mm.sup.2. Alternatively, sintering is made with the addition of ReAlO.sub.3 or Re.sub.3 Al.sub.5 O.sub.12 (wherein Re is a rare-earth element) in Japanese Patent Laid-open No. 60-186469. A bending strength thereof is around 70 kg/mm.sup.2 even at a room temperature. In Japanese Patent Laid-open No. 4-130062, the addition of a Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3 -based sintering aid such as YAG results in a sintered body of higher than 100 kg/mm.sup.2 at a room temperature. The strength is, however, around 80 kg/mm.sup.2 at 1200.degree. C.
As described above, the grain boundary phase is a glass phase, so that the strength, especially the strength at a high temperature, of the sintered Si.sub.3 N.sub.4 body is reduced significantly. With this respect, various trials have been made to improve the strength at a high temperature by means of crystallizing the glass phase. For example, in Japanese Patent Publication No. 52-45724, a small amount of TiO.sub.2 is added as a crystallization promoter and the sintering is conducted with SiO.sub.2 as well as Al.sub.2 O.sub.3 used as the sintering aids, after which the heating treatment is conducted at 700.degree.-1400.degree. C. to precipitate crystals such as cordierite and cristobalite. This provides the one having a bending strength of 60 kg/mm.sup.2 at 1200.degree. C.
In addition, Japanese Patent Laid-open No. 3-122055 discloses such a process that fine crystals of Si.sub.3 N.sub.4.Y.sub.2 O.sub.3 or Si.sub.2 N.sub.2 O are precipitated by using a heat treatment program in which it is held at 900.degree.-970.degree. C. during being cooled after Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3 are added, and then the temperature is increased to 1200.degree.-1300.degree. C. The bending strength of the resultant sintered body is only around 67 kg/mm.sup.2 at 1200.degree. C.
Japanese Patent Laid-open No. 3-199165 describes to control a cooling rate after sintering and thereby to form an Re.sub.2 O.sub.3 (wherein Re is a rare-earth element) --SiO.sub.2 -based grain boundary phase in which Si.sub.2 N.sub.2 O crystals are precipitated. The bending strength thereof is, however, 83-99 kg/mm.sup.2 at an ordinary temperature and is 51-63 kg/mm.sup.2 at 1400.degree. C. Japanese Patent Laid-open No. 4-231379 describes to form Si.sub.2 N.sub.2 O crystals into a grain boundary phase by means of heat treatment at 1200.degree. C. after the sintering, which allows production of such a sintered Si.sub.3 N.sub.4 body that has a strength of around 100 kg/mm.sup.2 at a room temperature and of around 63 kg/mm.sup.2 at 1400.degree. C. Japanese Patent Laid-open No. 5-294731 describes that a sintered body can be obtained that has a strength of 50 kg/mm.sup.2 or higher at 1400.degree. C. and a toughness value of 5 or larger in K.sub.IC can be obtained by forming a composite crystal phase of Re.sub.2 O.sub.3 (wherein Re is a rare-earth element) and ZrSiO.sub.4 by a heat treatment at 1500.degree. to 1700.degree. C. Japanese Patent Laid-open No. 5-330919 describes to form fine crystals based on Al-Yb-Si-O-N or Al-Er-Si-O-N in the crystal phase through the nucleation at 850.degree.-1050.degree. C. and the crystal growth at 1100.degree.-1500.degree. C. to obtain a sintered body having a bending strength of around 60 kg/mm.sup.2 at 1300.degree. C.
In addition, Japanese Patent Laid-open No. 5-58740 describes to precipitate an oxide crystal phase based on Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3 -SiO.sub.2 such as Y.sub.2 Si.sub.2 O.sub.7, Al.sub.5 Y.sub.3 O.sub.12, or Al.sub.6 Si.sub.2 O.sub.13 and thereby to obtain a sintered Si.sub.3 N.sub.4 body having a bending strength of around 50 kg/mm.sup.2 at a room temperature by means of a method in which MgO or TiO.sub.2 is added as a nucleation agent and the sintering is conducted with Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 as the sintering aids, after which a sintered body is nucleated at 850.degree.-1050.degree. C. and crystal-grown at 1200.degree.-1300.degree. C.
As mentioned above, various trials have been made to improve the strength at a high temperature and the toughness by means of combining the components and amount of the sintering aid(s), and crystallization treatment conditions, which is still insufficient to satisfy high temperature properties required for tools in recent years.